Transmission of urgent messages in frequency hopping system for intermittent transmission

ABSTRACT

A radio transmission system including many radio transmitters using frequency hopping carriers to intermittently transmit very short messages indicative of status of stimuli associated with the transmitters. The transmitters transmit transmissions independently of a receiver receiving the transmissions and independent of each other. In operation, radio transmitters transmit messages at varying frequencies at time intervals that can be varied as well. The frequency and time intervals are varied according to patterns that can be determined individually for each transmitter. A receiver holds data indicative of the future transmission frequency and time for each transmitter and updates the data based on the time and the content of the received messages. In addition, a simple method is provided to generate a very large number of orthogonal frequency-time hopping sequences that are individual for each transmitter and based on the transmitter ID. The problem of transmission of urgent messages is solved by transmitting urgent transmissions at transmission opportunities having precise time and frequencies that, advantageously, are in relations with the routine transmissions.

FIELD OF THE INVENTION

The present invention relates to telemetry in general, and, moreparticularly, to a system in which a plurality of transmitterswirelessly transmit transmissions for processing by one or morereceivers.

BACKGROUND OF THE INVENTION

Some wireless telemetry systems (e.g., burglar alarms, fire alarms,power utility meters, leak detectors, environmental monitoring,temperature control, etc.) comprise many transmitters that periodicallyor sporadically transmit messages to one or more receivers. In thesesystems, each transmitter is located at a different place and transmitsmessages that indicate the status of sensors associated with thetransmitter. A centrally located receiver receives messages from eachtransmitter.

Normally, the transmitters transmit messages that are as short asfeasible and with the interval between the transmissions as long asfeasible. This is advantageous for two reasons. First, it minimizes theaverage current drain in the transmitters, which are typically batteryoperated. Second, short and infrequent transmissions lower theprobability that data is lost as a result of collisions that occur whentwo or more transmitters transmit at the same time.

However, if an urgency is detected by the sensor associated with thetransmitter, the transmitter transmits immediately in order to notifythe receiver of the urgency as soon as possible.

Typically, a telemetry system transmits at a single frequency, and thusis susceptible to narrowband interference and signal loss due to aphenomena known as “multipath fading.” As a consequence, the reliabilityof such systems is compromised or, conversely, the transmitted power hasto be increased to overcome the fading, which results in larger powerdrain and shorter battery life. Furthermore, there usually areregulatory limits that restrict such transmitter power and thus limitthe possible compensation for multipath fading by merely increasing thetransmission power.

Because the multipath effect is highly sensitive to the frequency of thetransmitted carrier, a system using multiple frequencies (e.g., afrequency hopping spread spectrum system, etc.) has the potential toeliminate these drawbacks. However, frequency hopping systems require along acquisition time and are typically used in two way communicationapplications in which all the devices can be synchronized bycontinuously synchronizing with one master device or with each otherusing a variety of synchronization methods suitable for such case.

In other cases, to ease the synchronization problem, there are employedreceivers that can simultaneously receive signals at many frequencies bymaking the receiver broadband or by using several receivers at the sametime. Generally, these receivers suffer from performance degradation orhigh cost or both which makes them undesirable for low cost applicationsthat require high reliability such as security systems.

One serious problem that must be addressed in battery operated systemsconcerns battery life, and, therefore, it is advantageous if a telemetrysystem could be devised that shortened a transmitted messages preamble.A short preamble, however, makes it difficult for the receiver and thetransmitter to become and stay synchronized. This problem is exacerbatedin some systems, such as security alarms, that require some messages tobe conveyed to the system immediately without waiting for the scheduledtransmission time.

SUMMARY OF THE INVENTION

Some embodiments of the present invention comprise a frequency hoppingreceiver that acquires and maintains synchronization with a plurality oftransmitters, which enables the transmitters to omit the transmission oflong preambles. This is advantageous because ti lowers the averagecurrent drain in the transmitters and, consequently, lengthens theirbattery life. Furthermore, some embodiments of the present invention areadvantageous in that they provide improved reliability in the presenceof multipath fading, interference and jamming. And still furthermore,some embodiments of the present invention are capable of eliminating theeffect of persistent collisions that occur when two or more transmitterstransmit at the same time in the same channel for a prolonged period.

The illustrative embodiment of the present invention is afrequency-hopping wireless telemetry system comprising: (1) one or morereceivers, and (2) one or more transmitters, each of which receive inputfrom one or more sensors. The transmitters intermittently transmit veryshort messages indicative of status of the sensors associated with thetransmitters. Each transmitter includes a time interval generator toestablish the time interval between successive transmissions, afrequency synthesizer-modulator to generate a modulated radio frequencycarrier signal wherein the frequency of the carrier changes in responseto programming the synthesizer by digital data, a reference frequencyoscillator to provide a frequency reference from which the synthesizerderives carrier frequencies and, advantageously, from which the timeinterval generator derives its timing, and a transmitter control logicactivated in response to pulses from the time interval generator or asensor signal indicating an abnormal condition. When activated, thetransmitter control logic activates and programs the synthesizer so thatthe transmitter carrier frequency is changed according to a frequencyhopping algorithm, provides digital data indicative of the sensor statusand advantageously battery status, and modulates the carrier with theprovided data. The receiver includes a frequency selective radioreceiver circuit, programmable by digital data, to receive anddemodulate a transmitted carrier when the frequency of the receivercircuit is programmed according to the frequency of the carrier, and areceiver control logic means to process demodulated data, to providesystem interface responsive to the received messages, and to program thefrequency of the frequency selective receiver circuit. The control logicincludes a receiver timer to measure the elapsing time, and a pluralityof memory registers to hold digital data indicative of (a) the time ofthe next transmission occurrence for each transmitter and (b) thefrequency of the next transmission occurrence for each transmitter. Inoperation, the control logic sequentially compares the data content ofthe time registers with the data content of the timer and if thetransmission is due from a transmitter, the control logic programs thefrequency selective radio receiver circuit according to the data contentin the frequency register associated with said transmitter, attempts todecode the demodulated signal, modifies the content of the time registerby a number representative of the time interval between the successivetransmissions for said transmitter and modifies the content of thefrequency register according to a predetermined algorithm for saidtransmitter.

In accordance with the illustrative embodiment of the present inventionthere is provided a method of transmission in the system so as toimprove reliability of the system in the presence of multipath fadingand interference, the method is based on varying the transmissionfrequency for each transmitted message and varying the time betweenconsecutive messages. The frequency variations provide frequencydiversity and are effective against multipath fading as well as singleof multiple narrowband interference. The time variations are effectiveagainst periodic impulse interference. In combination, the frequency andtime variations provide immunity for a wide variety of signalimpairments and interference including multipath fading, wide andnarrowband interference, impulse noise and deliberate jamming.

In accordance with the illustrative embodiment of the present inventionthere is provided a method of minimizing the effect of collisions, themethod is based on selecting the transmission frequencies in sequencesthat are different for each transmitter, wherein transmitter frequencysequence depends on the transmitter ID number or other number derived orassociated with the transmitter ID. In addition, in the illustrativeembodiment, the transmitter ID number or other number derived orassociated with the transmitter number is included in the transmittedmessage, so that, upon reception of a single message from a transmitter,the receiver can determine what is the next frequency for thistransmitter, and thus achieve synchronization with this transmitter.

In accordance with the illustrative embodiment of the present inventionthere is provided another method of minimizing the effect of collisionsthat can be used alone or in conjunction with the third aspect of thisinvention, the method comprising randomizing the time interval betweentransmissions individually for each transmitter and a receivercompensating for the time interval changes.

In accordance with the illustrative embodiment of the present inventionthere is provided a simple method to generate a very large number offrequency-time hopping sequences. The method produces sequences that areorthogonal, thus eliminates possibility of persistent collisions evenwhen large number of transmitters are used. In addition, the methodrequires identical circuit in each transmitter and the actual sequencethat is produced is selected by the transmitter ID or other numberassociated with the transmitter ID, thus making it convenient formanufacturing. Also, the method enables to produce a very large numberof frequency-time sequences based on a single short PN generator whosestate can be instantly recovered by a receiver based on just onereceived transmission, thus aiding the receiver in obtainingsynchronization with a transmitter whose ID is known. At the same time,because of a very large number of possible sequences that can begenerated, it is difficult to obtain synchronization if the transmitterID is not known, which makes the system immune to interception andjamming.

In accordance with the illustrative embodiment of the present inventionthere is provided a method that enables such a system to convey theinformation about an abnormal sensor condition as soon as the conditionoccurs regardless of the transmission period of the associatedtransmitter. The method is based on transmitting urgent messages on oneof a plurality of transmission opportunities that allow the receiver totune to the right frequency at the right time to check if there is anytransmission pending form a transmitter. The urgent messages are,advantageously, synchronized to the routine messages, in effect, makingthe routine transmission an aid for the receiver in synchronization andtracking of the transmission opportunities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmitter according to a advantageousembodiment of the present invention.

FIG. 2 is a block diagram of a receiver according to a advantageousembodiment of the present invention.

FIG. 3 is a block diagram of an example of illustrative embodiment of asequence generator used to determine the individual frequency sequences.

FIG. 4 is a block diagram of an example of a illustrative embodiment ofa sequence generator used to determine the individual frequency-timesequences.

FIG. 5 is a block diagram depicting the frequency hopping systemincluding many transmitters and a receiver.

FIG. 6 a is a block diagram depicting an example of routinetransmissions.

FIG. 6 b is a block diagram depicting an example of routinetransmissions and transmission opportunities.

FIG. 6 c is a block diagram depicting an example of transmission ofurgent transmissions.

FIG. 6 d is a block diagram depicting another example of transmission ofurgent transmissions.

FIG. 6 e is a block diagram depicting an example of relating routinetransmissions with transmission opportunities based on a decimation of abasic sequence.

FIG. 6 f is a block diagram depicting an example of relating routinetransmissions with transmission opportunities based on two differentbasic sequences.

DETAILED DESCRIPTION

Referring to FIG. 5, the illustrative embodiment of the presentinvention depicts telemetry system comprising: receiver 401 and aplurality of transmitters 402, 403, 404 and 405. Receiver 401 is awireless receiver as described in detail below that includes a systeminterface 410 through which the receiver can be connected to interfaceequipment (e.g., a controller, a computer, etc.).

Each transmitter includes an interface or a sensor or an operation to bemonitored, and each transmitter intermittently transmits short messagesto the receiver that indicate, among other things, the status of theassociated sensor(s) or device(s) or operation(s). The transmitters arenot connected to each other and do not receive messages back from thereceiver. I.e. a transmitter does not have the capability to receivesignaling, timing or data from other transmitters or from the receiver.Furthermore, a transmitter transmit messages when it needs to withoutany regard to whether the other transmitters are transmitting andwithout regard to the receiver.

Referring to FIG. 1, the illustrative embodiment of a transmittercomprises a reference frequency crystal oscillator 6 to produce a stablefrequency on line 26, a time interval generator 2 establishing a timebase to produce pulses on line 28 activating the transmitter, afrequency synthesizer-modulator 4 to produce a radio frequency carriermodulated by modulation data fed to the synthesizer via line 24 whereinthe frequency of the carrier is programmed to a desired value viaplurality of lines 14, transmitter control logic 8 to activate andprogram the synthesizer-modulator 4 via plurality of lines 14 when thelogic is activated by a pulse from the time interval generator or by anabnormal signal indication on a sensor signal input line 18, anamplifier 10 to amplify the radio carrier provided by the synthesizerwhen the amplifier is activated by the control logic 8 via line 16, andan antenna 12 to radiate the power delivered by the amplifier. Thecontrol logic 8 includes a frequency and time data memory register 20 tohold information used to determine the time and the frequency of nexttransmission, and a sensor interface circuit 22 to accept the sensorsignal and detect an abnormal signal condition, and to convert thesensor signal to a digital format suitable for transmission. Thetransmitter logic also includes a storage means 30 to store atransmitter ID number to differentiate this transmitter from othertransmitters. The transmitter control logic, in some systems, can berealized based on microprocessor, in some other systems, a specializedapplication specific integrated circuit—ASIC component may be used.

In operation, during the time between transmissions, the transmitter isin a standby mode in which the amplifier 10 and synthesizer-modulator 4are not active and, advantageously, the control signals turn off thepower from these circuits in order to minimize the standby current ofthe transmitter. The transmitter control logic 8 is in a standby mode inwhich the most of the circuits are inactive and some or most of thecircuitry can be powered down with the exception of the circuitssupporting critical functions; (a) the sensor interface circuit 22 thatdetects an abnormal signal condition and produces a binary signal thatis logically combined with the signal 28 produced by the time intervalgenerator so that when either a pulse or abnormal condition occurs therest of the transmit logic circuitry is activated or powered up, (b) thefrequency and time data memory 20 that has to retain the data during theperiod between transmission and consequently either it has to be anonvolatile type or it has to be powered up during the period betweentransmissions. Upon activation, the control logic 8 determines theactivation source by reading signals 28 and 18.

When the logic 8 is activated by a pulse 28 from the time intervalgenerator the following sequence of events occurs. First, the logicreads the frequency data memory and produces a data packet that includesthe sensor status, the transmitter ID number and other data such asbattery status. Then, the logic activates and programs thesynthesizer-modulator 4, activates the amplifier 10 and sends the packetto the modulator via line 24. After completion of each transmission, thetransmitter logic sets the transmitter in the standby mode untilactivated again by a pulse on line 28 or a sensor abnormal conditionindicated on line 18.

In the advantageous embodiment the transmission of a packet can berepeated a predetermined number of times at separate frequencies,wherein the number of repetitions is chosen according to applicationneeds and, wherein the frequencies are determined by the transmitterlogic according to an algorithm described later in details. This way, itis possible for the receiver to receive some repeated packets even ifthe other packets are lost due to frequency selective fading caused bymultipath or due to interference. Similarly, a single packet can besplit into several pieces and each piece transmitted at a separatefrequency. In yet another embodiment, it may be advantageous to use morethan one carrier at the same time to improve reliability, however thiswould require a more complex transmitter.

When a sensor abnormal condition occurs, the sensor interface circuit 22produces an active level of the signal indicative of the sensor abnormallevel which activates the transmitter via a combinatorial logic circuitthat combines the sensor abnormal level signal with the pulses from thetime interval generator. When activated this way, the transmittercontrol logic 8 produces a data packet that includes the sensor status,then the logic activates and programs the synthesizer-modulator 4,activates the amplifier 10, and sends the alarm packet to thesynthesizer-modulator.

In the advantageous embodiment, the transmission of such alarm packet isrepeated a number of times according to the methods described later indetails.

After the transmission sequence is completed, the control logic disablesthe signal indicative of the sensor abnormal status so that an abnormalsensor status can activate the control logic. Then, the control logicputs the transmitter in the standby mode until activated by a pulse fromthe time interval generator. When subsequently activated, thetransmitter control logic performs the usual transmission sequence butthe data packets include information that the sensor condition isabnormal if the condition persists. When the abnormal conditionsubsides, the signal indicative of an abnormal status is enabled so thata subsequent occurrence of an abnormal condition can activate the logicand trigger a new alarm transmission sequence. Thus, normal operation isrestored.

Although FIG. 1 shows a specific illustrative embodiment, it is apparentthat various modifications may be realized such as including more thanone sensor, placing different ID in each sensor, or even placing theentire control logic in a sensor, or combining transmitter and sensor,etc.

In the advantageous embodiment, the sequence in which the frequenciesare used is different for different transmitters. The following is thedescription how this is accomplished in the advantageous embodiment.Each transmitter includes a pseudo random sequence generator or pseudonoise—PN generator, wherein a pseudo random sequence generator is basedon a linear feedback shift register, and wherein some outputs of theshift register are fed back to an EX-OR (Exclusive OR) gate whose outputis connected to the register input. For a certain combination of theoutputs that are fed to the EX-OR gate, the shift register can produce asequence that has 2^(N)-1 bits, wherein N is the length of the shiftregister. Such a sequence is called a maximum length sequence.Alternatively, if all the outputs of the shift register are taken at atime, then a pseudo random sequence of 2^(N)-1 numbers is created,wherein all the numbers have N digits and each number differs from allthe other numbers in the sequence; the numbers range from 1 to 2^(N)-1.For example, a generator based on a 3-bit shift register produces asequence consisting of 7 3-bit numbers. The numbers range from 1 to 7.Such PN generators are well known to the skilled in the art.

Referring to FIG. 3, the pseudo random sequence generator 203 consistsof a shift register 205 and EX-OR gate 204. The shift register 205 iscomposed of three stages 221, 222, and 223 having three outputs Q₀ 211,Q₁ 212 and Q₂ 213 respectively. The feedback is taken from outputs Q₀and Q₂. The three least significant bits of the transmitter ID {i₂, i₁,i₀} 201 are combined with the output of the pseudo random sequencegenerator {Q₂, Q₁, Q₀} using EX-OR gates 208, 207, and 206. The resultcan be used to indicate the frequency or frequency channel or frequencyindex {f₂, f₁, f₀} 202 indicative of the frequency over which thetransmission will occur.

Assuming that the initial state of the shift register is binary 111(decimal 7; Q₂=1, Q₁=1 Q₀=1), the produced sequence is {7, 3, 5, 2, 1,4, 6}. These numbers are then combined with the last three bits of thetransmitter ID using bit by bit EX-OR operation; i.e. the last bit ofthe transmitter ID (i₀) is combined with the last bit of the randomnumber (Q₀), etc. This way produced new sequence has numbers rangingfrom 0 to 7 the order of which depends on the last three bits of thetransmitter ID. Thus, a 8 distinct (permuted) sequences of numbers arecreated.

For example, if the last digits of the transmitter ID are 000, then thefrequencies are selected in the order 7, 3, 5, 2, 1, 4, 6, i.e. thesequence is not altered. If the last three digits of the transmitter IDare 001, then the frequencies are selected in the order 6, 2, 4, 3, 0,5, 7; if the last three digits of the transmitter ID are 010, then thefrequencies are selected in the order 5, 1, 7, 0, 3, 6, 4; etc. Notice,that each newly created sequence is not, strictly speaking, apermutation of the basic sequence because in each new sequence onenumber is converted to 0; e.g. in the first example 1 was converted to 0in the process, in the second example 2 was converted to zero. However,for the purpose of this application the operation is regarded and calleda “permuting operation” or a “permutation”, or “permutation process”,etc. Similarly, the resulting sequence is called “permuted sequence”.

In the illustrative embodiment, the sequence length depends on thenumber of frequencies used by the system. For example, if the availablebandwidth is 26 MHz and the frequencies are separated by a 100 kHzinterval, then there are 260 frequencies—channels available fortransmission. I.e. the sequence length can be 255 by using an 8-bitshift register. By processing the output of the PN generator with 8transmitter ID bits, a 255 different frequency sequences are obtained.

It is apparent that any two sequences are quite different even thoughthe ID number is changed only on one position. In fact, the sequencesare orthogonal, i.e., for any two sequences, a coincidence of twonumbers occurs only once for the entire period of the PN generator.

This is advantageous since it lowers the probability of persistentcollisions that may happen if two or more transmitters transmit at thesame time and at the same frequency for a prolonged time. It should bestressed that using the sequences as described ensures that thepersistent collision between any two transmitters is not possible sincethe frequencies in any arbitrary pair of sequences do not coincidepersistently regardless of the relative shift of the sequences.

For each transmitter, the future frequency can be predicted based onjust one partially received message because each message includes thetransmitter ID based on which the receiver can determine the content ofthe frequency index generator. To see how this can be done one needs tonotice that the permutation process according to the illustrativeembodiment is reversible: i.e., if the same ID digits are applied againto the frequency index, the PN generator state is obtained. If thereceiver receives a least a part of a message from a transmitter, thefrequency index is known. It is only necessary to know the digits of thetransmitter ID. The transmitter ID digits are included in the message.Advantageously, they are placed at the end of the message so that thereceiver can recover them even if it starts the reception of thetransmission in the middle of the message.

In some applications, this number of sequences may not be sufficient.For example, in many applications, the transmitter IDs are generatedsequentially in the factory and embedded in the transmitter circuit. Itwould not be convenient or sometimes not even possible to make sure thatall the transmitters to be installed on one premise or in geographicalproximity would produce orthogonal sequences. In such cases the numberof sequences can be extended to a larger number using other techniques,some of which were extensively studied and are described in theavailable literature. The number of available permutations of a sequencethat has 127 numbers is 127! (≈3E213). Even if a small subset of all theavailable permutations is used, the will be a large and adequate numberof frequencies produced. This way, the manufacturer can still embedsequentially produced ID numbers and the transmitters could still beused without regard to the sequences that they produce. However, themethod described above is advantageous for its simplicity and the uniqueproperties or orthogonality of all sequences. The degree oforthogonality indicates how many hits (frequency agreements) there maybe between two sequences upon any relative cyclic shift of thesequences. In a perfect design, for any two sequence that use the sameset of frequencies, there would be only one hit. I.e., if upon anycyclic shift of two sequences, a position is found in which the samefrequency is present in both sequences, then the frequencies in allother positions would differ. The sequences produced in a manner asdescribed in the advantageous embodiment are orthogonal in that sense.Although perfect orthogonality is not necessary for proper operation ofthe system, it is desirable since it reduces the probability of lostpackets due to collisions. However, it should be apparent that otherways of permuting the sequences could be created. Another advantage ofthe advantageous method of permutation is that the permutation processis reversible as previously described. This property enables thereceiver to synchronize based on the transmitter ID embedded in thetransmitted messages without a need for additional information bitsrelated to the PN generator status. Since the transmitter ID is normallyrequired in the transmitted messages, the advantageous method does notrequire any overhead in the transmitted messages.

In order to preserve the property of orthogonality and zero overhead asdescribed above and enlarge the number of transmitter ID universe thefollowing advantageous method is employed.

Normally, the time intervals between transmissions are controlled by aquartz crystal and, ideally their nominal values are the same for alltransmitters, however in the advantageous embodiment, the time intervalsare perturbed by small time increments to further randomize thetransmission events and lower the probability of persistent collisionswith other transmitters as well as avoiding an intentional orunintentional pulsed interference. The transmitter control logic canaccomplish this by programming the time interval generator via line 27(FIG. 1) according to a predetermined algorithm. The information aboutthe current status of the algorithm may be included in the transmittedpacket to aid the receiver operation.

In the advantageous embodiment, the method of determining the timeinterval perturbation is based on a similar technique as described inconjunction with the frequency index generation, wherein the randomsequence is used to alter the time interval between transmissions. I.e.,each time a transmission is performed, a new number is generated andused to determine the time interval between the current and the nexttransmission. Wherein, the time randomization is accomplished byprocessing the output of the PN generator used for the frequency indexwith bits of the transmitter ID. The processing is done as follows.

Referring to FIG. 4, the frequency index is produced by the PN generator203 outputs 213, 212 and 211 and transmitter ID bits 201—{i₂, i₁, i₀}processed with EXOR gates 208, 207, and 206 to produce index digits202—{f₂, f₁, f₀} as described previously in conjunction with FIG. 3. ThePN generator output is further processed with transmitter ID digits302—{i₅, i₄, i₃} by the AND gates 308, 306 and 304 and by an EXOR gate310. The output of the gate 310 taken one bit at the time is a shiftedreplica of the output of the PN generator e.g. output 211 or 212 or 213.Where the relative shift depends on the transmitter ID digits 302. Theoutput of the gate 310 is then fed to a shift register 312 whose outputs323, 322 and 321 are shifted replicas of the PN generator outputs 213,212, and 211 respectively. When taken three digits at the time, thesequence produced at the output of the shift register is a shiftedreplica of the output of the PN generator. For example, if the PNsequence produced is order {7, 3, 5, 2, 1, 4, 6}, and bits i₅, i₄, i₃are 011 then the shifted sequence is {4, 6, 7, 3, 5, 2, 1}; if the bitsi₅, i₄, i₃ are 101 then the shifted sequence is {2, 1, 4, 6, 7, 3, 5}.This way, a total of 7 shifted sequences are produced (000 input is notallowed). The shifted sequences are further processed with bits i₈, i₇and i₆ of the transmitter ID by EXOR gates 318, 316, and 316 to producepermutations of the shifted sequences at the outputs 333—{t₂, t₁, t₀} ina manner identical to the previously described in conjunction withfrequency index generation. This way, each shifted sequence can bepermuted in 8 different ways creating total 7*8=56 shifted-permutedsequences. The shifted and permuted sequences are used to producevariations of the time between consecutive transmissions. In theillustrative embodiment, the numbers from a sequence are multiplied by adT=2*Tm and added to the nominal time between transmission—TBT. Where,Tm is the nominal message transmission time. Advantageously, dT isrounded to the nearest discrete multiple of a basic time measure unitused by the control logic. If the permuted PN sequences are used asfrequency indexes and the shifted-permuted sequences are used torandomize the time between transmission, then there are created8*7*8=448 sequences that are time-frequency orthogonal in the sense thatif two sequences coincide at one frequency and time, they will notcoincide for any other frequency and time for the entire PN generatorperiod. This is based on merely 3-bit generator of the illustrativeexample! Of course, if a longer shift register is used for the PNgenerator, a far greater number of sequences are created. In theillustrative embodiment, an 8-bit generator is used as describedpreviously. This results in over 16E6 orthogonal time-frequencysequences. Enough to relieve the manufacturer and applications fromtransmitter ID management other than sequential numbering of allmanufactured transmitters. Of course, for a 8 bits shift register, 8bits of the transmitter ID are used to obtain sequence permutation forfrequency index, similarly, 8 bits are used for shifting and another 8bits are permuting the shifted sequence to obtain the time delayvariations.

The essence of this method is that in addition to two apparentdimensions of variability present in the form of permutations offrequency and time sequences, there is a third dimension added: i.e. thephase relationship variability between the frequency and time sequences.This rapidly increases a number of distinct orthogonal frequency-timesequences with increasing length of the basic PN generator as evidencedby the illustrative example. While it is possible to use other kinds ofbasic sequence and to use other ways of transforming the numbers of thebasic sequence to obtain new sequences, the added new dimension hasseveral advantages as evidenced in the illustrative embodiment.

The permutation process as described is an example of a more generalprocess of transformation that transforms a set of numbers into anotherset of numbers (that may differ in size). It should be apparent thatalthough a transformation resulting in the permutation as described isadvantageous, other transformations may be used to derive frequency-timepattern based on the described principle.

It should also be apparent that in some implementations the order inwhich the shift and the second transformation (permutation) is performedmay be reversed without altering the essence of the method.

Another advantage of the illustrative embodiment is that thepermutations and shifting of the sequences can be performed byprocessing (transforming) one number of the sequence at the time, thuseliminating the need to store and manipulate the entire sequence. I.e.,the permuted or shifted sequence numbers are produced one at the time asneeded based on numbers from the basic sequence that are also producedone at the time as needed.

Note, that this advantageous way of producing the frequencies does notrequire any overhead in the transmitted messages for the synchronizationpurpose other than the transmitter ID that is normally required any way.This is because the receiver can instantly recover the PN generatorstatus based on just a single received message. As described previously,the receiver can infer the status of the 8-bit generator based on thereceived frequency index and the transmitter ID number. I.e. the messagecontains the information about the 8-bit generator without explicitinclusion of the generator status bits in the message. In theillustrative embodiment, after the frequency index is obtained for atransmission, the time index is obtained by filling the shift registerin the steps of storing the PN generator status, clocking the PNgenerator and shift register N times, and restoring PN generator status.This way, the content of the shift register 312 is not required by thereceiver to obtain synchronization because the time index depends on thefuture content of the PN generator that can be easily duplicated in thereceiver based on the present content. Therefore, the receiver can stillsynchronize with a transmitter based on one received message and themessage does not need to include any overhead of synchronization.

In an alternative implementation, a second PN generator synchronizedwith the first PN generator may be used to produce the time variationswherein an information about the second generator phase is included inthe transmitted message to aid the synchronization. Note, thatsynchronization of the first and the second generator in the transmitteris extremely important since the essence of the idea is that the cyclicshift of the second sequence is provided in respect to the referenceprovided by the phase of the first sequence. Only this way the resultingfrequency-time hopping sequences produced in different transmitters aredistinct and orthogonal.

Although, the described implementation based on a single generator isadvantageous since it results in a simpler implementation and loweroverhead leading to a longer battery life, the two generatorimplementation can be modified to ensure low overhead as follows.

Both, first and second PN generators produce basic sequences whoselength is 2^(N)-1 and 2^(M)-1, wherein N and M are the lengths of therespective shift registers in both generators. In order to provide forsynchronization between both sequences, each sequence is extended by onebit by inserting one “0” bit at a predetermined place in the sequence.The advantageous place is after N-1 or M-1 “0” bits in the respectivesequences. This way the lengths become 2^(N) and 2^(M) respectivelywhich ensures that both sequence lengths are related by a power of 2(i.e. 2, 4, 8, etc.). Now, it is possible to ensure that both sequencesare always in the same phase relation; e.g. after initial reset whichsets the generators in a predetermined state, both generators areadvanced at the same time. This way, they will return to the exactinitial state after the full period of the longer sequence. Of theparticular interest is the case of the time generator producing a longersequence than the frequency generator. In some applications, there is alimited number of frequency channels available, however there is still aneed to produce a large number of frequency-time orthogonal sequences.In such case, a longer time sequence can be used to expand the number ofpossible frequency-time sequences. For example if the frequencygenerator shift register has N bits and the time generator shiftregister has M bits, then the total number of sequences is2^(N)*(2^(M)-1)*2^(M) as shown in the preceding examples. Each time M isincreased by one, the number of frequency-time sequences is enlargedapproximately by a factor of four resulting in a rapid increase of thenumber of sequences with the increase of the time generator shiftregister length. Also, the synchronization requires a small overheadbecause the receiver can infer the frequency generator state and needsonly the state of the time generator. However, if the time generator isin precise phase lock with the frequency generator, the transmitter doesnot need to send the actual time generator state. Instead, thetransmitter needs to include the information to remove the uncertaintycreated by the time sequence period being multiple of the frequencysequence period. E.g., if the time sequence is two times longer, thereceiver needs to know if the time generator is in the first half or thesecond half of the sequence to determine the exact state of the timegenerator. In this particular case, this information requires only onebit to be included in the transmitted messages. Of course, more bits arerequired if the time sequence is longer, e.g. if the time sequences is 4times longer than the frequency sequence, two bits are required; for 8times longer sequence 3 bits are needed, etc.

The described method (with one or two generators) produce a large numberof time-frequency orthogonal sequences in a simple and systematic waythat enables the sequence selection by the transmitter ID and requireszero (or very small) overhead for synchronization. A system using alarge number of time-frequency orthogonal sequences as described has anadvantage of immunity to multipath fading, pulsed and frequencyselective interference including intentional jamming, as well as lowprobability of self interference due to persistent collisions that mayoccur when two or more transmitters transmit messages on the samefrequency and at the same time for a prolonged period. A large number ofproduced frequencies enables the manufacturer and the system operatorsnot to be concerned with the management of sequences for all thetransmitters. Instead, each manufactured transmitter can produce aunique sequence that can be easily replicated in the receiver based juston the transmitter ID.

It is to be understood that the random frequency selection as describedabove and the time perturbation can be used together or in separation toachieve immunity to collisions. I.e. (a) a fixed frequency pattern forall transmitters and random time perturbation patterns individual foreach transmitter can be used, or (b) a fixed time interval betweentransmission or fixed time perturbation pattern and random frequencyselection individual for each transmitter can be used, or (c) frequencyand time changes can be combined to enhance the system performance atthe expense of complication.

In the advantageous embodiment, both the transmission frequency and thetime interval between transmissions are individually randomized for eachtransmitter by the transmitter ID bits.

It is also to be understood that one does not have to use thetransmitter ID bits to individually predetermine the frequency and/ortime patterns for each transmitter. In an alternative design, a randomseed can be generated in the transmitter, for example just after reset,and used in lieu of the ID number to modify the frequency and timepatterns. If the random seed has many bits, the probability ofgenerating the same pattern by two transmitters in the system is verysmall. However, this solution is considered inferior because it requiresthat additional steps are taken to associate the random seed number withthe transmitter ID. In addition, this solution requires a good truerandom number generator that produces numbers with roughly the sameprobability in order to prevent frequent repetitions of some numbers.Also, although the probability of sequence repetition is small, it maybe necessary to include an additional step in the process installing thetransmitter to reject a seed number that is already used by anothertransmitter. All this increases the complexity and the cost andpotentially makes the installation more difficult.

It is also to be understood that the illustrated method and itscomponents such as generators, registers, gates, etc., can be realizedin various forms of hardware some of which may include ASIC, orsoftware, or their combination.

Referring to FIG. 2, the receiver includes a reference frequency crystaloscillator 126 to produce a stable reference frequency on line 128 forthe receiver circuits, a frequency selective radio receiver circuit 100whose frequency is programmable via lines 116, to receive and demodulatea frequency modulated carrier when the frequency of the frequencyselective receiver circuit is programmed according to the frequency ofthe carrier, and a receiver control logic means 130 to processdemodulated data, to provide system interface lines 140, responsive tothe received data, and to program the frequency of the frequencyselective receiver circuit. The control logic includes a receiver timer132 establishing a time base to measure the elapsing time. The controllogic also includes: (a) a plurality of ID memory registers 134 to holddigital data indicative of ID numbers for each transmitter that belongsto the system, (b) a plurality of time memory registers 136 to holddigital data indicative of the time of the next transmission occurrencefor each respective transmitter, and (c) a plurality of frequency memoryregisters 138 to hold digital data indicative of the frequency of thenext transmission occurrence for each respective transmitter. In theadvantageous embodiment, the registers are organized such that anarbitrary register i 151 of the plurality of ID memory registers 134associated with a transmitter whose ID number is n, is associated withregister i 152 of the plurality of time memory registers 136 andregister i 153 of plurality of frequency memory registers 138, whereinsaid registers 152 and 153 hold data associated with said transmitter n.The frequency selective radio receiver circuit 100 includes a RF bandpass filter 104, an amplifier 106, an IF bandpass filter 110, a mixer108, limiter-discriminator circuit 112 and frequency synthesizer 114.The RF band-pass filter selects only the desired frequency bandallocated for the transmission, the mixer mixes the incoming signal withthe signal produced in the frequency synthesizer and produces an IFfrequency (Intermediate Frequency). The IF frequency is filtered in anarrow band filter 110 whose bandwidth is selected according to thechannel bandwidth. The limiter discriminator demodulates the signal andproduces baseband DATA signal 120 and an RSSI signal 118 indicative ofthe received signal strength. The DATA signal 120 and the RSSI signal118 are converted to binary signals by A/D converters 124 and 122respectively and fed to the control logic 130. The presentedarchitecture of the frequency selective radio receiver circuit 100 isknown as a superheterodyne FM receiver, it is very well known and itdoes not require additional explanation. The transmitted message data isextracted from the DATA signal 120 digitized by the A/D converter 124using one of the many well-known methods for signal processing and doesnot require additional explanation.

In the advantageous embodiment, the frequency registers 138 hold foreach transmitter the state of the PN generator used by the transmitterto produce the frequency indexes and time variations. If thesynchronization is obtained with a given transmitter, the state of thePN generator is identical to that in the transmitter. In theillustrative embodiment, the time registers 136 hold numbers—time ofnext transmission—for each transmitter representing the state of thereceiver timer 132 at the time the next transmission is due from atransmitter.

In operation, the receiver control logic 130 sequentially compares thedata content of the time registers 136 with the data content of thereceiver timer 132 and if the transmission is due from a transmitter,the control logic programs the frequency selective radio receivercircuit 100 according to the data content in the frequency register 138for this transmitter, attempts to decode the demodulated signal, changesthe content of the time register based on the number representative ofthe time interval between the transmissions for this transmitter andchanges the content of the frequency register according to apredetermined algorithm for this transmitter. I.e. the frequency and thetime registers are updated each time a transmission is due regardlesswhether the packet was received successfully. The new content of thefrequency register is determined according to the algorithm for thefrequency use by the transmitters.

The new content of the time register is calculated based on the currentcontent of the receiver timer and a number representative of the timebetween the current transmission and the next transmission for thistransmitter, wherein said number is calculated based on the nominalvalue of the time between the transmissions and adjusted by the pseudorandom perturbation performed according to the previously describedalgorithm. In addition, said number is corrected by a correction factorbased on the measured difference between the transmitter time base andthe time base of the receiver, wherein said difference is determined ina manner described later in details. In the advantageous embodiment, thenumbers representative of the time base differences are stored in thetime registers 136 separately for each transmitter and are independentfrom the numbers representing the time of the next transmission, i.e.the time registers are split to hold two independent numbers.

It should be noted that even if crystal oscillators are used in thetransmitters and the receiver to control the timing, the erroraccumulated during the time between transmissions can be significantcompared to the packet time. For example, if the nominal period betweenthe transmissions is 100 seconds and the crystal frequency error due totolerance and temperature changes is +/−20 ppm (parts per million) forthe transmitter and +/−10 ppm for the receiver, then the error may be aslarge as 3 ms. If the time for the transmission of one packet is 5 ms,then the error is significant. In reality, such tight tolerance isdifficult to achieve and expensive and therefore in many applications amuch bigger error will be accumulated. In order to minimize the timeerror accumulated during the long time between the transmissions, thereceiver can store the time difference between the ideal and the actualtime of the packet reception and use the difference to predict moreaccurately the next transmission time. For example, if the timerresolution is 0.25 ms, then the next transmission time can be predictedwith accuracy 0.25 ms, providing that the temperature does not changeappreciably over 100 s period. This represents an improvement of anorder of magnitude. I.e. the receiver can program its frequency 0.25 msin advance to each new frequency, examine it for the duration of thepacket, then program to the next frequency and so on.

During the acquisition, when the time error is not known, the receiverneeds to tune to the first frequency at least 3 ms in advance. Then, thereceiver monitors the received signal by observing the RSSI signal 118and DATA signal 120. If during the next 6 ms no valid signal is present,the receiver programs to the next frequency 3 ms in advance and so on.Of course, if worse tolerances are used, the time the receiver dwells ona single frequency expecting the next transmission is proportionatelylonger.

Some embodiments of the present invention relates to the reading ofelectric utility meters. In such embodiments, the transmittersadvantageously use the power-line as a frequency reference from whichthe time intervals between transmissions are derived. The advantage ofthis arrangement is that all transmitters time drifts in the sameproportions relative to the receiver. This can ease the receiver task ofacquisition as well as tracking.

However, in the illustrative embodiment, to ease this acquisitionproblem, the receiver includes a frequency error detection means and amethod as described below.

In the advantageous embodiment, the receiver includes a frequency errordetection means 142 that is advantageously implemented as a simpledigital counter, in order to detect the frequency error in the receivedsignal in respect to the receiver reference frequency by measuring thefrequency error of the intermediate frequency signal 111. In addition,in the transmitter, the transmitted carrier frequency and the timeinterval generator timing are derived from the same source and in thereceiver, the receiver frequency and the receiver timer are derived fromthe same reference. In operation, the receiver can measure the frequencydifference between the transmitted carrier and the receiver frequencyand use the measured error to determine the difference between thetransmitter reference frequency and the receiver reference frequencybased on just one partially decoded message. The frequency differencemeasurement is accomplished in the following way. Assuming that thetransmitter frequency accuracy is +/−20 ppm and the receiver is +/−10ppm, the carrier frequency is 915 MHz and the IF frequency is 10.7 MHz,the absolute maximum error between the receiver frequency synthesizerand the received carrier can be as much as 2760 Hz(915E6*20E-6+925.7E6*10E-6). I.e., the resulting If frequency is offsetfrom its nominal value by this amount. This represents 260 ppm of thenominal IF frequency. An ordinary frequency counter with a time baseaccuracy determined by the receiver crystal oscillator, i.e. +/−10 ppmcan detect this error and measure it with good accuracy. The accuracyshould be better than +/−110 Hz ({fraction (1/26)} of the maximumerror). Based on the measured frequency error, the relative frequencyoffset is calculated and the time correction factor for each transmitteris adjusted accordingly. For example, if the measured error is +1380 Hzthen the relative frequency error is approximately equal to +15 ppm. Ifthe nominal value of the time interval between two consecutivetransmissions is 100 seconds, then the required correction is +1.5 ms ifthe receiver uses high injection, i.e. the frequency of the synthesizerin the receiver is nominally equal to the received frequency plus the IFfrequency, and −1.5 ms if low injection is used.

In the advantageous embodiment, the time base correction factor storedfor each transmitter is also used to adjust the center frequency of thereceiver and thus aid the reception of the transmitted packets, thuslowering the requirements for the length of the preamble included ineach packet for the purpose of carrier and data timing acquisition. Thisis accomplished by adjusting the receiver frequency momentarily justprior to the reception of the packet from a transmitter from which apacket is due.

In the advantageous embodiment, when a transmitter is powered up, forexample after a battery replacement, it enters a power-up mode duringwhich a predetermined number of packets are transmitted on a number ofselected frequencies. In the power-up transmission sequence, each packetincludes a number that indicates how many packets the transmitter hastransmitted in this mode or how many packets the transmitter willtransmit in this mode before entering a normal mode of operation. Thisway, the receiver can synchronize with the transmitter just after asingle packet reception by calculating when the first transmission willoccur in the normal mode.

In operation, the receiver scans the selected frequencies during thetime when it is not occupied with the scheduled reception from thetransmitters or checking the time registers. Also the receiver scans allthe available frequencies in addition to the selected frequencies.During the scan, the receiver uses RSSI signal to detect if there is anenergy transmitted on a current frequency; if so, then the receivermeasures a predetermined unique properties of the modulated carrier. Ifthe energy is not present or the unique property is not valid, thereceiver will quickly proceed to examine the next frequency. Otherwisethe receiver will stay on this frequency and try to decode the message.This way all the selected frequencies are examined several times persecond ensuring that the receiver can receive a power-up message. Also,the scan of all available frequencies is fast; the synchronization canbe regained faster and more reliably because the receiver will not wastemuch of the time for an examination of very weak or spurious signals.

In the advantageous embodiment, the transmitter ID numbers for eachtransmitter stored in the receiver ID memory registers 134 are acquiredand stored by the receiver during a process of log-in. Each newtransmitter to be logged-in is placed in a relatively close proximity tothe receiver and then powered up. A very high level of the receivedsignal ensures that the new transmitter signal is not mistaken foranother transmitter. A successful log-in is confirmed by the receiverusing an audio or a visual indicator that can be included in thereceiver or in the system controller connected to the receiver viasystem interface 140. The receiver may reject the transmitters that cancause persistent collisions, i.e. if its ID number has the last 24digits identical to another transmitter already present in the system.

One of the fundamental concerns in some types of such systems is theneed for transmission of urgent messages. Such a need arises in e.g.security alarm systems. In such systems, the regular transmission occurat relatively long time intervals in order of 100 to 200 seconds or evenmore. According to the description given earlier in this application,these transmissions can occur at constant or varying time intervals.They are referred to as “routine” transmissions. Typically, they merelycarry status information indicating that the transmitter is operativeand may include some additional information, e.g. battery status.Therefore, these “routine” transmissions are also referred to as“status” transmissions. It may be convenient to think of them as“scheduled” transmissions.

Another example of such a system is a heating and air-conditioningsystem. In such a system, there may be several “thermostats”, eachincluding a frequency hopping transmitter. Each thermostat sends“routine” transmissions, at intervals, including temperature readingdata, to a receiver controlling the heating and air-conditioning system.

Referring to FIG. 6 a, the status transmissions are transmitted at timeintervals T_(i) 601, T_(i+1) 602, T_(i+2) 603, etc. In the figure, theyare shown to be equal although they do not have to be. Each statustransmission is shown to occurs at the beginning of each time interval,although this is, of course, arbitrary. The transmissions and intervalsare indexed for reference. Accordingly, the transmission 604 havingindex i is followed by transmission 608 having index i+1 and which inturn is followed by transmission 612 having index i+2 etc. As indicatedbefore, each transmission can use one or more frequencies at the sametime or one at the time. The time intervals may also be variedadvantageously as previously described. The indexes are also used todetermine the frequency (or frequencies) for the transmission and, inanother advantageous implementation, the duration of the intervalbetween transmissions. Advantageously, as indicated before, thesedeterminations may be done individually for each transmitter. For theillustration of the basic idea, let's assume that the time intervals arefixed, and that the indexes indicate the frequency used for thetransmission.

When an urgency occurs, e.g. an alarm condition, an urgent message mustbe transmitted to the receiver promptly; waiting 200 seconds or even 10seconds for the next scheduled time for the next transmission is notpractical. In general, such a message is referred to as an “urgentmessage” and a transmission that carry such message is called an “urgenttransmission.” It may be convenient to think of such messages as“unscheduled”, although, as is shown later, this is not a precisedescription. Suppose that the alarm occurs in the middle of the interval602. It is evident, that in order to transmit the alarm message duringthe next scheduled transmission, the transmitter needs to wait forseveral tens of seconds.

Note, that the distinction between the message and the transmission ismade to indicate that they may not be the same. E.g. one transmissionmay convey only a fraction of a message or more than one message.

Similarly, in a heating and air-conditioning system, when a user adjuststhe temperature on a thermostat, he expects almost immediate action,e.g. the furnace fires-up or the air-conditioner turns on. This requiresthat the transmitter sends an “urgent” message to the receiver much inthe same fashion as in the alarm systems.

In a frequency hopping system according to this application, thefundamental problem is that the receiver tracks time and frequency forall the scheduled transmissions, but without knowing when and where toexpect the alarm message, it has difficulties to receive suchtransmission. It does not help to know what the next frequency may bewithout knowing the time because this frequency may be not usable, forexample due to multipath fading. It is therefore necessary that thetransmitter transmits at more than one frequency. I.e. change thetransmission frequency, perhaps even several times. However, a changehas to occur at some time. If that time is unknown, the receiver cannotfollow to the new frequency. This is a difficult problem.

In one possible solution, the transmitter repeats transmission of thealarm transmissions at many frequencies for a predetermined duration ora predetermined number of times. The receiver scans all the frequenciesall the time to find out if there are any such transmissions, in hopethat it will eventually stumble on one of such transmissions. Theinterception of such transmission is probabilistic, i.e. a matter ofchance. In many cases, this is problematic. Consider the followingexample. Assume that the message time is 5 ms, that there are 128frequencies, and that receiver needs 1 ms to scan one frequency todetermine if a message is being transmitted.

In this example, the probability that a single message is intercepted,at least in part, is 5/128, i.e. very small. Many transmissions areneeded to make this probability acceptably high. For example, if a 100messages are transmitted, the probability that all are missed is stillvery high—approximately 20%. Almost 300 messages are needed to make theprobability less that one in a million.

This solution has several drawbacks, one of them is excessive batterydrain, especially if the alarm events occur frequently. For example, ina door monitoring transmitter, were each closing or opening the dooractivates the transmitter.

In an alternative implementation, to increase the probability of asingle transmission interception, the alarm messages may be transmittedat a smaller number of specially assigned alarm frequencies. Forexample, if 8 frequencies are used then 15 transmissions are sufficientto achieve less than one in a million probability that all thetransmissions are missed.

However, this method also has drawbacks; for example: the assignedfrequencies are easy to interfere with by an intentional jammer, theimmunity to impairments such as multipath and interference is reduced,there may be regulatory limitations that may not allow transmission ofexcessive energy on selected frequencies, etc.

Accordingly, the new method described here solves all these problems andprovides a solution to the fundamental problem without apparentdrawbacks with ease and elegance.

The following is a description of an illustrative example of animplementation of the new method.

For the purpose of transmission of urgent messages that need totransmitted before the next status transmission, there are establishedmultiple transmission opportunities at certain times. Theseopportunities are established in each time interval between statustransmissions. When such opportunity has a frequency index assigned toit based on which a transmission frequency is assigned to thisopportunity, the transmission opportunity becomes more than simply atime slot. The transmission opportunities, then, have two coordinates:time and frequency. They are referred to as “transmission opportunities”having “opportunity time slots” and “opportunity frequency index”. Itshould be also obvious that in some applications, advantageously, aspreading code can be used, e.g. as in direct sequence spread spectrum,or multicarrier spreading. In such a case, it can be said that anadditional coordinate is introduced, that is the spreading code.

For the purpose of this specification, the term “transmissionopportunity” is defined as: (i) time, or (ii) frequency, or (iii) code,or (iv) any combination of (i), (ii), and (iii) at which a transmissionmight occur.

Advantageously, the opportunity time slots are at relatively muchshorter time intervals, e.g. 250 ms compared to the status transmissionsthat are at intervals of 100-200 seconds.

In operation, the receiver checks each opportunity for transmission bytuning to the right frequency at the right time. Normally, thetransmitter is not concerned with the time slots and the transmissionopportunities. It simply transmits the status messages when due.Consequently, normally, there are no transmissions transmitted at theseopportunities. However, when an alarm condition occurs, the transmittercomputes the time and the frequency index of the next transmissionopportunity. Then, the transmitter transmits the urgent message at thenext transmission opportunity, i.e. at the right time and frequency.This transmission can be repeated at several transmission opportunitiesto improve reliability. Of course, in both, the receiver and thetransmitter, the time and frequency index of the next opportunity arecomputed based on some convenient reference, e.g. the time and frequencyof the last status transmission. Obviously, the computation is doneaccording to an algorithm that is consistent for both the transmitterand the receiver.

It should be apparent, that as long as the receiver maintainssynchronization with the status transmissions, it is able to receiveevery transmission at every opportunity providing that the transmittedsignal is not impaired by fading of interference. This is a vastimprovement.

In another implementation, advantageously, the routine transmissions aretransmitted at some of the transmission opportunities, which makes itconvenient for the receiver to maintain synchronization and to computethe time and frequency for the future transmission opportunities.

For convenience, similarly, one can think of the frequency and timecoordinates for the routine transmissions as “routine transmissionopportunities”, although this is not the same as in the case of urgenttransmissions because the transmission opportunities are rarely used bythe urgent transmissions as opposed to the routine transmissions thattypically always occur at their “opportunities”.

For illustration, consider FIG. 6 b.

In all the following figures, for illustration purpose, thetransmissions are marked as boxes with continuous line, and thetransmission opportunities in which the transmissions do not occur areshown as boxes with broken lines.

The transmission opportunities are shown in each of the time intervals:605, 606, 607 in interval T_(i), 609, 610, 611 in interval T_(i+1), 613,614, 615 in interval T_(i+2), etc. They are depicted as indexed timeslots. The indexes are indicative of the frequency (or frequencies to beused). It should be noted that although there are only threeopportunities shown, in reality there may be hundreds of them in asingle interval. For example, if the interval has 200 seconds and theopportunities are every 250 ms, then there are 800 opportunities in thistime interval. In the illustrative example, the indexes are assignedaccording to a very simple rule: for each opportunity, the index isincremented by one starting from the last status transmission, and resetat the end of the interval back to the value of the last statustransmission. Then, the index is incremented by one for the next statustransmission and the entire process is repeated, etc. I.e. theopportunities following status transmission with index i, have thefrequency index i+1 (605), i+2 (606), i+3 (607), the opportunitiesfollowing status transmission with index i+1, have the frequency indexi+2 (609), i+3 (610), i+4 (611), etc. The status transmissions 604, 608,612, 616 have indexes i, i+1, i+2, i+3, respectively, exactly as in FIG.6 a.

This way, the status transmissions are transmitted with indexes that areincremented by one for each consecutive transmission, and the indexesfor each opportunity are easily referenced to the last transmission.There is no particular significance to this assignment, and many otherassignments are possible, some of which are highlighted later. In theillustration, the opportunities are separated by interval Ts 619, thatis shown to be equal for each pair of opportunities. In advantageousimplementation, these intervals can be varied in a similar fashion aspreviously described in conjunction with frequency and time hopping. Ofcourse, a different advantageous indexing may be used to obtainorthogonality of frequency-time sequences for each transmitter. This hasbeen described in details earlier and does not need additionalexplanation.

FIG. 6 c illustrates what happens when an alarm event occurs. Assumingthat an alarm event 618 happened to occur some time after opportunity609 but before opportunity 610, it is shown that the alarm transmissionare performed at opportunities 610, 611, 613, and 614 following thealarm event. Optionally, the alarm message may also be transmitted atthe slot 612 at time and frequency scheduled for status message or thestatus transmission may include an alarm indicator. The alarmtransmissions are performed for a known duration Ta 620 or until a knownnumber of transmissions is performed. This simple way of indexing thestatus and alarm transmissions is advantageous for its simplicity andfor consistent indexing of the status transmissions regardless whetherthe alarm transmissions are performed.

FIG. 6 d illustrates another simple way of indexing. In this case, theindexes are assigned in such way that they are used consecutively foreach transmission including status and alarm transmissions. This isadvantageous for frequency usage to be evenly distributed over allfrequencies, but requires more careful receiver operation since it iseasier to lose synchronization with the status transmission sequence.

Of course, in both examples, the actual values of indexes depend on whenthe alarm event 618 occurs and on the duration or equivalently thenumber of transmissions in the time slots following the alarm event.

It should be apparent, that there can be many different ways ofestablishing the time slots and many different ways of assigningfrequency index to each slot. One can, for example, use a specialsequence that always starts alarm transmissions from the same index, orthe special sequence may run continuously advancing for eachopportunity, one might reference the slot time to the last transmittedmessage or to another time reference related to many previous and evenfuture transmissions, one may synchronize the special sequence to thestatus sequence or one may convey the alarm sequence status in thestatus transmissions, etc., etc.

One interesting example is described below for illustrative purpose.According to this implementation, one can create a continuous sequenceof indexed time slots and “decimate” them for the purpose oftransmitting the status transmission, i.e. skip over withouttransmission in most of the slots. Then, when alarm occurs, transmit thealarm messages more often (using different decimation factor, or notdecimating at all, i.e. transmitting more often). Advantageously, thedecimation factor can be used that is L+1, where L is the sequencelength. For example, if the sequence numbers are 0,1,2,3, etc., theindexes used for the status transmissions will be in the same order0,1,2,3, etc. This is illustrated in FIG. 6 e. There, the sequencelength is 3 and the numbers are is 0,1,2; the decimation factor is 4.This is a very elegant way to look at the idea and event to implementit. Of course, the decimation will work just as fine with any factorthat is n*L+1, where n is a natural number.

In FIG. 6 f, another implementation is illustrated, using twointertwined sequences that are different from each other; first used forscheduled (status) transmissions and the second used for alarmtransmissions. As mentioned before, they can be precisely synchronizedwith each other, so that the receiver can reproduce the second knowingthe phase of the first. Alternatively, the phase information of thesecond sequence may be included in the status transmissions.

All these variations are possible within the scope of the describedidea. One needs to recognize that these implementations are mostly amechanical variation of the basic idea as described.

The essence of the idea is that, firstly, the transmission opportunitiesfor urgent transmissions do exist at precise times, and secondly, thateach opportunity for transmission has a precise frequency index (andpossibly a spreading code) assigned, which allows the receiver tocompute the expected transmission time and frequency and to tune tospecific frequencies and only at precise times when the alarmtransmissions can be transmitted. Furthermore, advantageously, thetransmission opportunities are developed in relation to the routinetransmissions, that in effect provide synchronization for thetransmission opportunities. In other words, the urgent transmissions canonly occur at certain discrete coordinates that, advantageously, can berelated to the routine transmissions.

This method has several advantages. One advantage is that each alarmtransmission reception is no longer probabilistic. I.e. eachtransmission can be received (providing that the propagation conditionsand interference level permits it). Consequently, the necessary numberof transmissions is just one, although more than one can transmissionmay be utilized in order to compensate for impairments due topropagation and interference. Obviously, this saves battery and reducesinterference created by multiple transmissions. Another advantage isthat it is more difficult to intercept and jam the alarm transmissions.Yet another advantage is that the receiver uses only a fraction of itstime to track one transmitter. For example, if the slots are establishedon average every 250 ms, and the receiver needs 1 ms to check if atransmission is being performed, then the receiver needs to spend only 4ms in one second for one transmitter. This is equivalent to 0.4% of thetime. Also, the response is almost instantaneous. The sub-second delayis inconsequential in most practical applications.

For the purpose of this specification, the transmitter control operationcan be performed by one or more dedicated or shared logic or processoror any other computing devices, e.g. personal computer or other kind ofcomputer that may be integrated or external or even remote. Insofar asthe functions performed by these logic or processor or computing devicesrelate to the operation of the transmitter they are considered a part ofthe transmitter for the purpose of this specification. The same claim ismade for all the receiver functions.

In the illustrative embodiment described here, references are made toseveral elements such as generators, logic, registers, controloperations, etc. It is to be understood that various elements describedhere can be realized in several different forms including software andhardware in their various forms and combinations. E.g. the “logic” canbe a hardware such as a gate or memory element, or it can be a piece ofsoftware to perform a certain task. In the later case, logic simplymeans “intelligence”.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

It is to be understood that the above-described embodiments are merelyillustrative of the invention and that many variations may be devised bythose skilled in the art without departing from the scope of theinvention. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

1. A telemetry system comprising: a plurality of transmitters, each ofwhich is for transmitting intermittently and at various transmissionfrequencies: (a) routine transmissions, at first time intervals, and (b)urgent transmissions, in response to urgency, at transmissionopportunities at second time intervals, wherein each of said pluralityof transmitters is for transmitting independently of any receiver forreceiving any of said transmissions and independently of any of saidplurality of transmitters, and a receiver for holding, simultaneouslyfor each of said plurality of transmitters, data indicative of anexpected time and an expected frequency of at least one futuretransmission opportunity.
 2. The system of claim 1 wherein said receiveris for holding, simultaneously for each of said plurality oftransmitters, data indicative of an expected time and an expectedfrequency of at least one future routine transmission.
 3. The system ofclaim 1 wherein each of said plurality of transmitters includes, in atleast a portion of said routine transmissions, data for indicating atleast one of: (a) frequency sequence for controlling frequency of saidtransmission opportunities, and (b) time sequence for controlling timeof said transmission opportunities.
 4. The system of claim 1 whereineach of said plurality of transmitters includes, in at least a portionof said routine transmissions, data for indicating at least one of: (a)frequency of at least one future transmission opportunity, and (b) timeof at least one future transmission opportunity.
 5. The system of claim1 wherein said transmission opportunities are synchronized with saidroutine transmissions.
 6. The system of claim 1 wherein transmissionfrequency of said routine transmissions is controlled according to afirst sequence, and frequency of said transmission opportunities iscontrolled according to a second sequence, and said first sequence issynchronized with said second sequence.
 7. The system of claim 1 whereinsaid first time intervals are controlled according to a first sequence,and said second time intervals are controlled according to a secondsequence, and said first sequence is synchronized with said secondsequence.
 8. A method comprising: transmitting, by each of a pluralityof transmitters, intermittently and at various transmission frequencies:(a) routine transmissions, at first time intervals, and (b) urgenttransmissions, in response to urgency, at transmission opportunities atsecond time intervals; wherein said transmissions are independent of anyreceiver for receiving any of said transmissions and independent of anyof said plurality of transmitters, and holding, in a receiver,simultaneously for each of said plurality of transmitters, dataindicative of an expected time and an expected frequency of at least onefuture transmission opportunity.
 9. The method of claim 8 furthercomprising holding, in said receiver, simultaneously for each of saidplurality of transmitters, data indicative of an expected time and anexpected frequency of at least one future routine transmission.
 10. Themethod of claim 8 further comprising, including by each of saidplurality of transmitters, in at least a portion of said routinetransmissions, data for indicating at least one of: (a) frequencysequence for controlling frequency of said transmission opportunities,and (b) time sequence for controlling time of said transmissionopportunities.
 11. The method of claim 8 further comprising, includingby each of said plurality of transmitters, in at least a portion of saidroutine transmissions, data for indicating at least one of: (a)frequency of at least one future transmission opportunity, and (b) timeof at least one future transmission opportunity.
 12. The method of claim8 wherein said transmission opportunities are synchronized with saidroutine transmissions.
 13. The method of claim 8 wherein transmissionfrequency of said routine transmissions is controlled according to afirst sequence, and frequency of said transmission opportunities iscontrolled according to a second sequence, and said first sequence issynchronized with said second sequence.
 14. The method of claim 8wherein said first time intervals are controlled according to a firstsequence, and said second time intervals are controlled according to asecond sequence, and said first sequence is synchronized with saidsecond sequence.
 15. A telemetry receiver comprising: logic for holding,simultaneously for each plurality of transmission opportunities, dataindicative of an expected time and an expected frequency of at least onefuture opportunity, wherein each said plurality of opportunities is fora different one of a plurality of transmitters, and wherein each of saidplurality of transmitters is for transmitting intermittently, at varioustransmission frequencies: (a) routine transmissions, at time intervals,and (b) urgent transmissions, in response to urgency, at at least one ofsaid opportunities; wherein each of said plurality of transmitters isfor transmitting independently of any receiver for receiving any of saidtransmissions and independently of any other of said plurality oftransmitters, and a frequency selective circuit for receiving saidtransmissions.
 16. The receiver of claim 15 wherein said logic is,further, for holding simultaneously for each of said plurality oftransmitters, data indicative of an expected time and an expectedtransmission frequency of at least one future routine transmission. 17.The receiver of claim 15 wherein, in operation, for each of saidplurality of transmitters, said receiver changes frequency of saidfrequency selective circuit to said expected frequency of said at leastone transmission opportunity at such time relative to said expected timeof said at least one transmission opportunity to receive and demodulate,when it occurs, said at least one urgent transmission.
 18. The receiverof claim 15 comprising a frequency error detector to detect a differencebetween an actual and an expected transmission frequency of said routinetransmissions, wherein said receiver utilizes said difference todetermine an expected time of a future transmission opportunity.
 19. Thereceiver of claim 15 wherein said receiver detects a difference betweenan actual and an expected transmission time of said routinetransmissions, and wherein said receiver utilizes said difference todetermine an expected time of a future transmission opportunity.
 20. Thereceiver of claim 15 wherein, said receiver extracts, from at least aportion of said routine transmissions, data indicative of at least oneof: (a) pattern of frequency variations for said transmissionsopportunities, and (b) pattern of time interval variations for saidtransmission opportunities.
 21. The receiver of claim 15 wherein, saidreceiver determines at least one of: (a) time of at least one futuretransmission opportunity and (b) frequency of at least one futuretransmission opportunity based on data included in at least one routinetransmission.
 22. A plurality of telemetry transmitters, each of whichcomprises: a circuit for transmitting intermittently and at varioustransmission frequencies: (a) routine transmissions, at first timeintervals, and (b) urgent transmissions, in response to urgency, attransmission opportunities at second time intervals, and logic forcontrolling frequency and time for said transmission opportunities andsaid routine transmissions independently of any receiver for receivingany of said transmissions and independently of any other of saidplurality of transmitters.
 23. The plurality of transmitters of claim 22wherein said transmission opportunities are synchronized with saidroutine transmissions.
 24. The plurality of transmitters of claim 22wherein each of said plurality of transmitters includes, in at least aportion of said routine transmissions, data indicative ofsynchronization information for at least a portion of futuretransmission opportunities.
 25. The plurality of transmitters of claim22 wherein each of said plurality of transmitters controls transmissionfrequency and time according to a frequency-time sequence that isdifferent for each of said plurality of transmitters.
 26. The pluralityof transmitters of claim 22 wherein each of said plurality oftransmitters includes, in at least a portion of said routinetransmissions, data indicative of a sequence for controlling at leastone of: (a) frequency, and (b) time, for at least a portion of futuretransmission opportunities.
 27. The plurality of transmitters of claim22 wherein transmission frequency of said routine transmissions iscontrolled according to a first sequence, said frequency of saidtransmission opportunities is controlled according to a second sequence,and said first sequence is synchronized with said second sequence. 28.The plurality of transmitters of claim 22 wherein said first timeintervals are controlled according to a first sequence, and said secondtime intervals are controlled according to a second sequence, and saidfirst sequence is synchronized with said second sequence.
 29. Aplurality of telemetry transmitters, each of which comprises: a circuitfor transmitting intermittently and at various transmission frequencies:(a) routine transmissions, at first time intervals, and (b) urgenttransmissions, in response to urgency, at transmission opportunities atsecond time intervals, and logic for including in at least a portion ofsaid routine transmissions data indicative of at least one of: (a)frequency pattern for varying frequency for said transmissionopportunities and (b) time pattern for varying said second timeintervals; wherein each of said plurality of transmitters is fortransmitting independently of any receiver for receiving any of saidtransmissions and independently of any other of said plurality oftransmitters.
 30. The plurality of transmitters of claim 29 wherein saiddata is based on bits of transmitter identification.
 31. The pluralityof transmitters of claim 29 wherein said transmission opportunities aresynchronized with said routine transmissions.
 32. The plurality oftransmitters of claim 29 wherein each of said plurality of transmittersincludes, in at least a portion of said routine transmissions, dataindicative of synchronization information for at least a portion offuture transmission opportunities.
 33. The plurality of transmitters ofclaim 29 wherein each of said plurality of transmitters controlstransmission frequency and time according to a frequency-time sequencethat is different for each of said plurality of transmitters.
 34. Theplurality of transmitters of claim 29 wherein transmission frequency ofsaid routine transmissions is controlled according to a first sequence,and frequency of said transmission opportunities is controlled accordingto a second sequence, and said first sequence is synchronized with saidsecond sequence.
 35. The plurality of transmitters of claim 29 whereinsaid first time intervals are controlled according to a first sequence,and said second time intervals are controlled according to a secondsequence, and said first sequence is synchronized with said secondsequence.